1) Field of the Invention
This invention relates in general to the paper making industry. In one aspect, the invention relates to a method for monitoring specific filtration resistance in a continuous on-line manner. In another aspect, the invention relates to a method for monitoring streaming potential and specific filtration resistance simultaneously in a continuous on-line manner. In a further aspect the invention is directed to a method for predicting drainage on the paper machine.
2) Background Art
Paper makers have known for a long time that the paper machine is a dynamic changing system. It gives the illusion of being a steady state process but when closely examined from a water quality perspective, the water quality is always slowly changing. The reason for this is the long times to equilibrium because of the high residence time recycle loops in the process. Much fundamental understanding exists about retention and drainage on the paper machine but attempts in practical application have been unsuccessful because of the changing system. A fundamental property which is important to retention on the paper machine is the state of the charge on the fiber surface, and a fundamental property important to drainage is specific filtration resistance, hereinafter also referred to as SFR.
The trend has been away from laboratory measurements and toward on-line continuous measurements for both drainage and electrostatic charge.
Zeta potential is an electrokinetic property of particles suspended in an aqueous medium containing charged ionic species and is an expression of the charge developed en or adjacent to such particles. It has been recognized that the zeta potential of fibrous particles in the feed stock or furnish, used in paper making, has a considerable influence upon the paper produced therefrom. Zeta potential cannot be measured directly; however, it can be calculated from measurements of a related parameter known as streaming potential.
A general discussion of zeta potential and its relevance to paper making is provided in "Electro kinetics in Paper Making--a position paper" by R. A. Stratton and J. W. Swanson in TAPPI, 64 No. 1, page 79-83 (1981). A survey of various methods of measuring zeta potential, including those reliant upon measurements of streaming potential, is given in an article by H. J. Jacobson et al, in Colloid and Polymer Science 263; 3-24 (1985).
From these references, it can be seen that the zeta potential of feed stock or furnish, exiting the head box of a paper making machine, influences the quality of the paper produced by the machine, as a result of its significant effect during paper formation on the wire of the machine. It is known that, in principle, a high particle charge is necessary to ionically stabilize colloidal emulsions or dispersions. There is a mutual repulsion between charged particles of the same polarity which keeps the particles apart and thereby imparts stability to an emulsion or dispersion system. Thus, stable dispersions of clay, calcium carbonate or titanium dioxide can have a charge or zeta potential of -50 to -60 mV. or even higher. Cellulose pulp dispersions typically have a zeta potential in the range of -15 to -20 mV. Cationic charge neutralizing chemicals such as alum, quaternary amines and wet strength resins, can reduce the zeta potential of a paper making furnish to within the range of 0 to -8 mV. At these low values, the repulsive effect of the charge on the suspended particles is reduced to a negligible value and coagulation is maximized. Conversely, at higher zeta potentials the degree of coagulation is reduced as the repulsive effect is increased. Thus, properties such as first pass retention, formation, drainage, white water consistency and strength can be optimized for a particular type of paper by adjusting the zeta potential of its precursor furnish to an optimum. Early attempts to apply zeta potential technology were often unsuccessful because the system conditions were different by the time the measurement was taken in the laboratory. This difficulty led to the development of the Zeta Data.TM. on-line streaming potential instrument. It was the first commercially available on-line streaming potential device.
In the drainage measurement field a similar evolution has taken place. The standard of the industry to measure drainage is Canadian Standard Freeness (CSF) in North America, or Schopper Riegler Freeness (SR) in Europe. They are empirical tests which measure the drainage time of a fixed volume of a sample. They give a qualitative picture of how a stock drains but no theory exists which can extrapolate their values to drainage on the paper machine. The fundamental measurement which characterizes stock drainage and has a theoretical underpinning is specific filtration resistance.
Specific filtration resistance is a property of the slurry and the conditions under which the pad is formed. It is a basic inherent property of material being drained.
The idea to use SFR to characterize stock drainage was introduce in the 1950's by Ingmanson and refined into a workable concept by Springer and Pires in 1989. The concept is good enough to be used to predict drainage down a paper machine. With such a parameter as specific filtration resistance available, the paper industry can phase out it's dependence on Freeness. The need for on-line Freeness testing was acknowledged in that several companies market such devices which have had significant commercial acceptance. Specific filtration resistance was not used due to industry lack of familiarity with the theory because user-friendly apparatus did not exist.
It is evident from the foregoing that there is a need for a method which allows substantially continuous and accurate monitoring of the specific filtration resistance of paper making feed stock or furnish.
The paper maker should be able to tell the drainage of the stock prior to entering the head box and adjust the electrochemical conditions with an appropriate polymer if there are going to be drainage problems which would slow down the machine. The device would be useful for monitoring and controlling refining conditions. The instrument should be very useful for monitoring the new microparticulate techniques which are aimed at maximization of retention, drainage and formation. It is therefore an object of the invention to provide a method for monitoring specific filtration resistance in paper making feed stock or furnish. Another object of this invention is to provide a method for the continuous, simultaneous and accurate monitoring of streaming potential and specific filtration resistance of a primarily fibrous material in a fluid. A further object is to provide a method for the determination of specific filtration resistance in furnish in order to predict drainage characteristics and thereby be enabled to make appropriate changes in the physical or chemical composition of the furnish either by refining or the addition of chemicals. These and other objects will readily become apparent to those skilled in the art in the light of the teachings herein set forth.